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United States Department of Agriculture

Agricultural Research Service

Research Project: Agricultural Landscape, Pollinator Behavior and Gene Flow Risk

Location: Vegetable Crops Research

Title: Floral traits influencing plant attractiveness to three bee species: Consequences for plant reproductive success

Author
item Bauer, Austin
item Clayton, Murray
item Brunet, Johanne

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2017
Publication Date: 5/21/2017
Citation: Bauer, A.A., Clayton, M.K., Brunet, J. 2017. Floral traits influencing plant attractiveness to three bee species: Consequences for plant reproductive success. American Journal of Botany. 104(5):772-781. doi: 10.3732/ajb.1600405.

Interpretive Summary: Alfalfa plants require insect visitation to produce seeds. Floral display size and flower color are visual traits that can play an important role in attracting pollinators to plants. This study examines and contrasts the role of different visual traits, components of floral display size and flower color, on the attractiveness of alfalfa plants to three bee species, the alfalfa leafcutting bee, Megachile rotundata, the common eastern bumble bee, Bombus impatiens, and the honey bee, Apis mellifera. The relationship between plant attractiveness and seed set is also determined for each bee species. While leafcutting bees and bumble bees represent the two major managed pollinators in alfalfa seed-production fields, bumble bees frequently visit flowering alfalfa in Wisconsin. All three bee species were influenced by the number of racemes on a plant. Plants with fewer racemes were more likely not to be visited by a bee and plants with more racemes received more visits. The impact of flower color varied with bee species. Bumble bees were more attracted to plants with purple flowers and plants with yellow flowers were more likely not to receive any visits. The preference of flower color changed between years for honey bees with attraction to purple flowers one year and little impact of flower color the second year. Leafcutting bees were not affected by flower color. Overall, floral display size had a greater impact on bee attraction relative to flower color. The number of visits increased seed set for all three bee species with the steepest slope observed for leafcutting bees, followed by bumble bees and finally leafcutting bees. Therefore, for a given number of visits, seed set was greatest following visits by leafcutting bees and lowest after honey bee visits. Differences in tripping rates among bee species helped explain these patterns. These results indicate that management practices that increase floral display size will favor visits by all three bee species and greater attractiveness increases seed set. Moreover, the increase in seed set resulting from an increase in floral display size will be greatest with leafcutting bees. By identifying plant traits that increase attractiveness to different pollinators with a positive impact on seed set or yield, these results benefit the alfalfa seed-production industry, farmers, and plant breeders.

Technical Abstract: The ability to attract pollinators is crucial to plants that rely on insects for pollination. We examined and contrasted the role of floral display size and flower color in attracting three bee species to Medicago sativa and determined the relationships between plant attractiveness and seed set for each bee species. A zero-inflated Poisson regression model helped identify the plant traits that most affected pollinator attraction. Different components of floral display size were examined and two models of flower color were contrasted. Regression analyses determined the relationships between plant attractiveness and seed set for each bee species. The number of racemes per plant most consistently affected attraction to all three bee species. The impact of flower color varied with bee species and the choice of the flower color model often influenced whether color affected attractiveness. Floral display size had a greater impact on bee attraction relative to flower color. The number of visits increased seed set for all three bee species with the steepest slope observed for the alfalfa leafcutting bee, followed by the common eastern bumble bee and finally the honey bee. Variation in tripping rate among bee species helped explain the differences in slopes. Floral display size plays an important role in attracting bee species to alfalfa plants and the differential increase in seed set with floral visits among bee species suggests potential differences in the level of selection on floral display size and flower color exserted by each species.

Last Modified: 07/25/2017
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